Holocene tree immigration and the chironomid fauna of a small Swiss subalpine lake (Hinterburgsee, 1515 m asl)

Early Holocene reforestation by stone pine (Pinus cembra) and tree birch (Betula pubescens) took place ca. 500 years after the end of the Younger Dryas at Hinterburgsee, a small subalpine lake in the northern Swiss Alps. During the next ca. 3000 years the local vegetation consisted of open woodlands with many pioneer dwarf shrubs and herbs. The expansion of silver fir (Abies alba) at ca. 7400 calibrated radiocarbon years before present (cal. BP) and Norway spruce (Picea abies) at ca. 6000 cal. BP in Hinterburgsee's catchment led to a closing of the local forests with a successive decrease in erosion and a distinct change in Hinterburgsee's sediment composition. First signs of probably human-induced openings of the catchment forest are apparent at ca. 2500 cal. 13P, but it is not until the past ca. 800 years that pollen analysis suggests strong local and regional anthropogenic activity. The strongest and most abrupt changes in the Holocene development of Hinterburgsee's chironomid fauna took place at ca. 11500 cal. BP and at ca. 10 000 cal. BP, when parts of the alpine taxa that were dominant during the Younger Dryas disappeared from the lake. The first change is most likely related to the increasing temperatures after the end of the Younger Dryas, the second possibly to decreasing lake depth and increasing summer insolation in the early Holocene that may have led to warmer bottom water temperatures in the lake. No clear relationship between the changes in catchment vegetation and the development of the chironomid fauna was found. Possibly the increase in sediment organic matter associated with the denser catchment forests was responsible for a succession in the chironomid stratigraphy between 7000 and 4500 cal. BP. However, due to the high sedimentation rates in Hinterburgsee this trend could also be a consequence of, or be promoted by, the sediment infilling of the lake basin. This conjecture is supported by the increasing importance of Tanytarsus lugens-type, a chironomid taxon dominant in the extant chironomid assemblages of shallow mountain lakes in Switzerland. The only shift in the chironomid fauna that clearly parallels a change in catchment vegetation is found during the past ca. 800 years. We discuss possible reasons for the comparatively weak influence of catchment vegetation on the chironomid fauna of Hinterburgsee and the implications of our results for multi-proxy studies on past climate involving both palaeobotanical and chironomid-based reconstructions.